Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329, incorporated herein by reference in its entirety. The carbonylation catalyst contains rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms. Likewise, because the nature of the halide promoter is not generally critical, a large number of suitable promoters, most of which are organic iodides, may be used. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved.
In the operation of the process for the carbonylation of methanol to acetic acid on a continuous basis, a solution containing the soluble catalyst complex is separated from the reactor effluent and recycled to the reactor. However, with operation over extended periods of time, corrosion products dissolve from the vessels of the metallurgy stream, e.g., iron, nickel, molybdenum, chromium, and the like, and build up in the catalyst recycle stream. Such corrosion metals, if present in sufficient quantities, are known to interfere with the carbonylation reaction or accelerate competing reactions such as the water-gas shift reaction (carbon dioxide and hydrogen formation) and methane formation. Thus, the presence of these corrosion metal contaminants has an adverse effect on the process, in particular, a consequent loss in carbon monoxide productivity. Further, corrosion metals can react with ionic iodine thus making this component of the catalytic system unavailable for reaction with rhodium and causing instability in the catalyst system. In view of the high cost of the rhodium-containing catalyst, replacement of spent catalyst can be effected only at a prohibitive cost.
U.S. Pat. No. 8,242,040, herein incorporated by reference, teaches a process for the removal of corrosion metal contaminants from a carbonylation catalyst solution comprising an iridium and/or rhodium carbonylation catalyst, an alkali and/or alkaline earth metal and corrosion metal contaminants. The catalyst solution is contacted with a cation exchange resin having its active sites partially loaded with a sufficient amount of alkali and/or alkaline earth metal to maintain the concentration of said alkali and/or alkaline earth metal in the catalyst solution. The catalyst solution of reduced corrosion metal contaminant content is then recovered.
U.S. Pat. No. 5,466,876, herein incorporated by reference, teaches that corrosion metal contaminants are removed from a liquid composition comprising a carboxylic acid and/or an anhydride thereof, a rhodium carbonylation catalyst, and a carbonylation catalyst co-promoter by using a chelating resin selective for the removal of corrosion metals rather than carbonylation catalyst and co-promoter. Additional methods of removing corrosion metal contaminants are also disclosed in U.S. Pat. Nos. 4,985,383 and 5,124,290.
U.S. Pat. No. 4,894,477, herein incorporated by reference, teaches the use of a cation exchange resin in the lithium form to remove metallic corrosion products from a carbonylation catalyst solution which contains a rhodium component and a lithium component. The process described in U.S. Pat. No. '477 is particularly applicable to those processes which are useful for the carbonylation of methanol to acetic acid under low water conditions, such as those set forth in U.S. Pat. No. 5,001,259. U.S. Pat. No. '477 further teaches that while low water conditions improve the acetic acid purification/production process, as lithium concentrations in the low water conditions carbonylation reactor are increased to increase rhodium stability and as the water levels in the reaction system are decreased, the capacity of the ion exchange corrosion metal removal process per cycle is diminished.
Similarly, U.S. Pat. No. 5,731,252, herein incorporated by reference, teaches a process for treating low water content carbonylation catalyst solutions which contain a rhodium component and an alkali metal component to remove metallic corrosion products. The process comprises contacting the catalyst solution with an ion exchange resin, preferably in the lithium form, and a sufficient amount of water to decrease the concentration of alkali metal ions to optimize removal of corrosion metal products.
While the above-described processes have been successful in generally removing some corrosion metals using an ion exchange resin, the need exists for improving acetic acid yield by setting an iron threshold and removing iron when it is above the threshold.